Manhole covers made of cast iron make-up the bulk of access covers in urban streets around the world. These covers have a proven record of durability, strength, and functionality. The high density of iron results in a cover of substantial weight preventing cover displacement during flooding conditions and minimizing the occurrence of theft.
Numerous attempts have been made to employ non-metallic covers. For example, U.S. Pat. No. 5,123,776 issued to Lang et al. discloses a manhole cover having an outer surface fabricated from a plastic material while an interior chamber is filled with cement. U.S. Patent Application Publication No. 2008/0050175 by Brady et al. discloses a lightweight reinforced composite manhole assembly that includes a manhole cover and base, which may be formed by injection molding.
Another attempt by Consolidated Edison Company includes a removable rectangular steel cover with an epoxy cover. The cover comprised a ¼-inch elongated solid steel plate, the ends of which were disposed 3/16-inch below the upper end of L-shaped brackets. A plurality of ¼-inch thick spaced-apart longitudinally-extending ribs were welded to the bottom of the steel plate. A 3/16-inch layer of epoxy was disposed on the outer surface of the cover between the upper portions of the L-shaped brackets and even with the upper portions of the L-shaped brackets.
An earlier approach by the applicant of the present invention involved employing a cast iron manhole cover having a recessed pocket on the top. In particular, the cast iron manhole cover had a raised peripherally-extending border disposed around the pocket. When the cast iron manhole is positioned and supported on the lip of a manhole, the top surface of the border is disposed at the same lever as the top surface of the rim of the manhole, e.g., the ground or road surface. The recessed pocket was filled with a resin and glass fiber composite. The resin was also coated over the top surface of the peripherally-extending border.
There is a need for further manhole covers, and in particular, thermally and electrically insulating manhole covers.